1. Field
The presently disclosed subject matter relates to a video projection apparatus and its controlling method. The video projection apparatus can be used as a pico projector, a head mount display (HMD) unit, a head up display (HUD) unit and the like.
2. Description of the Related Art
A prior art video projection apparatus is constructed by a two-dimensional optical deflector as an optical scanner manufactured by a micro electro mechanical system (MEMS) device manufactured using a semiconductor process and micro machine technology (see: JP2012-530264A & US2010/0315552A1). Since the MEMS optical scanner is focus-free, the video projection apparatus can be small in size.
In the above-described prior art video projection apparatus, when rocking the MEMS scanner in a main scanning at a high resonant frequency, a digital phase locked loop (PLL) circuit is provided to receive a sense voltage from a sensor within the optical deflector and generate a phase signal phase-locked to the sense voltage.
In the above-described prior art video projection apparatus, however, since the MEMS scanner is susceptible to electromagnetic interference (EMI) noises and external noises by environmental factors such as temperature and humidity, the digital PLL circuit may not operate at an optimal frequency and phase, so that an optimum resonant frequency would not be realized. As a result, it is difficult to accurately control a projected view field.
Also, in the above-described prior art video projection apparatus, although the frequency and phase are locked, the amplitude of the sense voltage representing the flexing angle of a mirror of the MEMS scanner can not be monitored by the digital PLL circuit, it would be difficult to accurately realize an optimum resonant frequency. As a result, it is difficult to accurately control a projected view field.